Nonameric porphyrin assemblies - Formation and intra-assembly energy transfer reactions

Article abstract of DOI:10.1039/b408023h

The nonameric porphyrin assemblies constructed with the series of free base tetraphenylporphyrins Pⁿ having four pyrazine moieties linked with alkyl chains of different lengths, (CH₂)_n (n- 1, 5, 9, 17, 30), and dimeric [mesotetrakis(2-carboxy-4-nonylphenyl)porphyrinato]zinc(II), ZnP₂, show the effective light-collection effect and the typical Foerster-type energy transfer from ZnP₂ to Pⁿ.

Full text of DOI:10.1039/b408023h

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A R T I C L E
Nonameric porphyrin assemblies – formation and intra-assembly
energy transfer reactions
Ken Sasaki,a Kenji Sugou,a Koji Miyamoto,a Jyun-ichi Hirai,a Shigetaka Tsubouchi,a
Hiroshi Miyasaka,b Akira Itayaa and Yasuhisa Kuroda*a
a
Department of Polymer Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku,
Kyoto 606-8585, Japan. E-mail: ykuroda@kit.ac.jp; Fax: +81-75-724-7830;
Tel: +81-75-724-7830
Department of Chemistry, Graduate School of Engineering Science, Osaka University,
b
1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Received 28th May 2004, Accepted 9th August 2004
First published as an Advance Article on the web 6th September 2004
The nonameric porphyrin assemblies constructed with the series of free base tetraphenylporphyrins Pn having
four pyrazine moieties linked with alkyl chains of different lengths, (CH2)n (n = 1, 5, 9, 17, 30), and dimeric [meso-
tetrakis(2-carboxy-4-nonylphenyl)porphyrinato]zinc(II), ZnP2, show the effective light-collection effect and the
typical Förster-type energy transfer from ZnP2 to Pn.
the chemical processes occurring in multi-component, large-
Introduction
scale assemblies. In this article, we describe the spectroscopic
and kinetic characteristics of the series of nonameric porphyrin
assemblies that have different lengths of alkyl chains linking the
antenna porphyrins with the central acceptor porphyrin.
The photosynthetic light-harvesting assembly is one of the most
impressive molecular architectures in biological systems.1 The
highly efficient chemical reactions taking place in this assembly
have been recognised as one of the ideal chemical processes
and, therefore, diverse efforts to mimic this molecular system
have been made for a long period.2 Its gigantic but still well-
defined structure and highly refined chemical functions provide
an excellent example of the challenging subject not only for
biomimetic chemistry but also for synthetic chemistry, which
will lead to the exploration of new methodologies to construct
artificial large-scale molecular architectures or devices. It is inter-
esting to note that biological large-scale molecular architectures
are sometimes constructed by repeated combination of a large
number of smaller molecular elements by means of molecular
recognition based on reversible intermolecular interactions. One
typical example of such large-scale biological assemblies is the
antenna complex in the photosynthetic light-harvesting system
which inevitably requires many bacteriochlorophyll molecules
for its functions of efficient light collection over a wide area
range. For example, the recent model of the antenna system,
LH2, from Rhodopseudomonas acidophila suggests that 27
antenna bacteriochlorophyll molecules are regularly arranged
around the photoreaction centre. In spite of its gigantic struc-
tures, the total antenna system is constructed as a nonamer of
a single elementary unit element containing two protein sub-
units and three bacteriochlorophyll moieties. These structural
characteristics make the antenna systems ideal as the target for
the artificial self-assembling molecular architectures.3
Recently we reported the construction of nonameric and
heptadecameric porphyrin assemblies which show efficient light-
harvesting effects and energy transfer from antenna pigments to
the single central porphyrin in these assemblies.4 These systems
are constructed by using specific coordination of the pyrazine
moiety on the zinc porphyrin dimer which contains tight
hydrogen bonds between the carboxylic acid groups formed in
the non-polar solvent such as dichloromethane.5 It seems to be
clear that the systems have not only the original advantage of the
self-assembling architecture in which the large-scale molecular
system is automatically generated by simple mixing of necessary
components, but also another great advantage that systematic
modification of their structural elements may be easily attained
by rather small modification of the synthetic methods for
the compounds. The latter advantage is very important for
systematic investigations that clarify the mechanistic details of
Results
Preparation of central free base porphyrins bearing four pyrazine
moieties
Our previous investigations reveal that the main photochemical
characteristics of the present type of porphyrin assemblies are
susceptible to the distance between the component porphyrins
but not so sensitive to the nature of chemical bonds connecting
the components.4 For the purpose of systematic investigation
of the antenna effect, we designed a series of TPP derivatives
bearing the antenna positions connected with ether linkages
of different alkyl chain lengths. The synthetic scheme of the
present central free base porphyrins is shown in Scheme 1. These
simple synthetic procedures are clearly advantageous for the
preparation of a series of porphyrin derivatives that should be
constructed with the common core porphyrin and easily avail-
able alkylpyrazine components. Three bromoalkylpyrazines
having penta-, nona- and hepta-decamethylene moieties were
prepared by the reaction of the pyrazinyllmethyl anion with
the corresponding dibromoalkane.6 The shortest bromoalkyl-
pyrazine, 2-bromomethylpyrazine, was obtained from direct
bromination of methylpyrazine by NBS. Since this compound
is not so stable, the crude product after purification through
a short silica-gel column was immediately used for the next
reaction. The pyrazine derivative having the longest arm is
prepared by insertion of an ether linkage in the alkyl chain
because of the difficult availability of the corresponding pure
alkyl compound. All central porphyrins, P1, P5, P9, P17, and P30,
used in this work were prepared by the coupling reaction of
meso-tetrakis(p-hydroxyphenyl)porphyrin with the correspond-
ing bromoalkylpyrazine, which was catalysed by 18-crown-6.
Although the yield of P1 was extremely low, probably due to
the low purity of 2-bromomethylpyrazine, further synthetic
refinement was not attempted. The electronic and fluorescence
spectra of these porphyrins are essentially identical and no
special interaction between the porphyrin core and pyrazinyl
moieties was observed. The HRMS and NMR spectra of all
final compounds show satisfactory agreement with the expected
structures.
2 8 5 2
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 8 5 2 – 2 8 6 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Scheme 1
Formation of nonameric porphyrin assemblies
the ZnP2–P5 system is shown in Fig. 2. Although there is no
strict isosbestic point due to the absorbance of the titrant P5,
its disturbance for the titration is fairly small because of the
relatively low concentration of P5 during the experiment. The
resulting spectroscopic behaviour is quite similar to that of
the titration of ZnP2 with simple pyrazine derivatives reported
previously.5 The Q-band spectrum of the 4:1 mixture of ZnP2
The electronic spectra of P5, ZnP2 and the ZnP2·pyrazine
complex are shown in Fig. 1. All electronic spectra of P5, P9,
P17 and P30 are practically superimposable, which indicates
that the appended pyrazine moieties do not perturb the
spectra of the core TPP molecule, regardless of the length
of the linker alkyl chains. The basic characteristics of the
nonameric porphyrin assembly formation are examined by the
spectroscopic titration experiments of dimeric [meso-tetrakis(2-
carboxy-4-nonylphenyl)porphyrinato]zinc(II), ZnP2, with the
free base porphyrins, Pn. The typical result of the titration for
Fig. 2 UV/vis spectroscopic titration of ZnP2 with P5 in CH2Cl2:
[ZnP2] = 1.41 × 10−6 M, [P5] = 0, 0.825, 1.64, 2.46, 2.86, 3.27, 3.67,
4.07, 4.87, 5.66, 6.45, 7.23 × 10−7 M. The dashed line is the composite
spectrum of ZnP2·pyrazine (1.41 × 10−6 M) + P5 (3.52 × 10−7 M). The
inset shows absorption changes at 549 and 566 nm: () observed points,
(—) theoretical curves calculated by using eqn. (2) with K = 9.2 ×
Fig. 1 Absorption spectra of P5 (8.09 × 10−6 M), ZnP2 (3.03 × 10−6 M)
and ZnP2·pyrazine (3.01 × 10−6 M) in CH2Cl2.
107 M−1
.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 8 5 2 – 2 8 6 0
2 8 5 3

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and P5 is practically identical with the composite spectrum of
4 × ZnP2·pyrazine + P5, i.e. both spectra show identical kmax
values within 0.5 nm errors and identical e values over the
whole Q-band region within 10% error.7 Quite similar results
are obtained for P9, P17 and P30 assemblies. The stoichiometry
of the assembly is also confirmed by Job’s plot analysis shown
in Fig. 3 in which P17 is used as the central free base porphyrin.
The maximum complex formation is observed at 0.88 of the
molar fraction of monomeric ZnP which is consistent with
8/9 of the ideal value for the nonameric assembly. From these
observations, we concluded that P5, P9, P17 and P30 form the
1:4 assemblies Pn·(ZnP2)4, and that the electronic interaction
between porphyrins in these assemblies is not sufficiently
strong for it to seriously disturb the binding of ZnP2 at each
pyrazine site of Pn.8 Thus the present assembling processes
to form Pn·(ZnP2)4 are reasonably expected to consist of four
independent complexation processes of ZnP2 at each pyrazine
moiety of Pn, which have a practically identical binding
constant and a site-independent molar absorptivity change
during complexation of ZnP2 (Scheme 2). Thus the process is
analysed by using the theoretical model derived for the multiple
equilibrium system that is statistically corrected for the identical
multi-binding sites.9
For the system having four identical binding sites, the binding
constant K is defined by
[Pn •(ZnP )i ]
2
K =
i =1,2,3,4.
(1)
[ZnP ][Pn •(ZnP )i−1
]
2
2
Assuming the site-independent molar absorptivity ec for ZnP2
complexing with the pyrazine moiety, the observed difference in
absorption during the titration is expressed by
Abs = Absobs −εZnP [ZnP ]T
2
2
4K∆ε[ZnP ][Pn ]T
2
=
+ εPn [Pn ]
T
(2)
1+ K[ZnP ]
2
where De is ec−eZnP and the subscript T indicates the total
2
concentration of the corresponding porphyrins. The observed
data are in excellent agreement with eqn. (2) as shown in the
inset of Fig. 2. All binding constants for P5, P9, P17 and P30 thus
determined are of the order of 107 M−1,10 which indicates that
complex formation at these binding sites is very favourable and
is insensitive to the distance from the central porphyrin and the
apparent mutual proximity of the antenna porphyrins.
In contrast to these porphyrins having longer alkyl chains,
P1 shows somewhat different behaviour in the spectroscopic
examination as shown in Fig. 4, i.e. the observed spectrum
for P1·(ZnP2)4 is significantly different from that composed
of the spectra of P1 and the ZnP2·pyrazine complex. The
absorption band at 650 nm which is characteristic of P1
clearly shifts to 655 nm upon complexation. Furthermore,
the largest discrepancy is observed in the Soret region, where
the P1·(ZnP2)4 complex shows a clear split of the main Soret
absorption indicating that there are non-negligible electronic
interactions between the component porphyrins in the assembly.
These results suggest the possibility that the four independent
binding site model shown in Scheme 2 is no longer proper for
the P1 assembly system. The experimental data for this system,
however, are analysed by the same method applied for the other
systems, since the titration curve still shows a monotonous
single phase saturation and a sharp bend at the concentration
ratio of P1/ZnP2 = 1/4, similar to those for P5 shown in Fig. 2.
The resulting value of the binding constant for the P1 system is
ca. 1 × 107 M−1, which shows that the binding of P1 with ZnP2
is still strong enough, although the binding constant should be
considered as an apparent average value.
Fig. 3 The job’s plot for the ZnP–P17 system in CH2Cl2 at 298 K. The
total porphyrin concentration is 2.63 × 10−6 M. Note that the ratio of the
porphyrins is given in terms of the monomeric Zn porphyrin ZnP.
Fig. 4 Absorption spectra of P1·(ZnP2) (7.56 × 10−7 M) and
P1 (7.56 × 10−7 M). The composite spectrum is reconstructed from the
spectra of ZnP2·pyrazine (3.02 × 10−6M) + P1 (7.56 × 10−7 M), where
[P1] = 7.56 × 10−7 M.
Scheme 2 Assembly formation equilibrium processes for four
independent binding sites with an identical binding constant.
2 8 5 4
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 8 5 2 – 2 8 6 0

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Table 1 Fluorescence quantum yields of Pn and spectroscopic para-
meters of the assemblies Pn·(ZnP2)4a
Fluorescence spectroscopy and energy transfer reactions of the
assemblies
c
d
e
Assembly
Ub
r564
fa
fb
The apparent antenna effects of the present assemblies
are examined by fluorescence spectroscopy. The result of
fluorescence titration of ZnP2 with P9 is shown in Fig. 5. The
system is excited at 564 nm where the largest Q-band absorption
of the ZnP2·pyrazine complex is observed. The basic behaviour
is similar to that of the UV/vis titration. Upon addition of P9 to
the solution of ZnP2, the intensity of the fluorescence of P9 at
657 nm rapidly increases until the ratio of P9/ZnP2 = 1/4, where
the titration curves plotted for the fluorescence intensity show
a sharp saturation and further addition results in only a weak
emission increase proportionate to the concentration of excess
P9. In contrast, the fluorescence of the ZnP2·pyrazine moieties
at 609 nm rapidly decreases until the ratio of P9/ZnP2 = 1/4. The
observed emission under the saturation conditions is attributed
mainly to the fluorescence of P9 despite the existence of a large
excess of the ZnP2·pyrazine moiety that is the main initial
excitation species in the assembly. These observations indicate
effective energy transfer from the ZnP2 moieties to the central
free base porphyrin P9 in the assembly and the intermolecular
energy transfer is negligible under the present conditions. In
order to quantitatively analyse the fluorescence behaviour of
the present assembly, the spectra of the assembly and each
component were compared in detail. The fluorescence spectra
of P9, the ZnP2·pyrazine complex and the P9·(ZnP2)4 assembly
are shown in Fig. 6a. All spectra were measured at the standard
conditions of [P9] = 4.31 × 10−7 M and [ZnP2] = 1.72 × 10−6 M.
Since the spectrum observed for P9·(ZnP2)4 still contains the
fluorescence of the ZnP2·pyrazine moiety, quantitative
evaluation of this donor fluorescence intensity is expected
to give good apparent indices for the intramolecular energy
transfer in the assembly. The multiple regression analysis11 for
the observed emission of P9·(ZnP2)4 reveals that the spectrum is
well reconstructed by combination of the fluorescence F1 for P9
and F2 for the ZnP2·pyrazine complex in the form of faF1 + fbF2,
where fa and fb are the amplifying factor for the acceptor
emission and the quenching factor for the donor emission,
respectively. The reconstructed spectrum shows excellent
agreement with the observed fluorescence as shown in Fig. 6a.
The fluorescence of free ZnP2 is not detected in these analyses,
indicating that practically all ZnP2 molecules are bound to the
pyrazine site of Pn. Similar results are also obtained for the P5,
P17 and P30 assemblies. These parameters thus determined are
summarised in Table 1 together with the fluorescence quantum
yields of the free base porphyrins and the absorption ratio of the
P1
0.114
0.132
0.120
0.120
0.129
23
20
21
21
18
23
20
16
12
9
0.10
0.19
0.24
0.32
0.51
P5
P9
P17
P30
a In CH2Cl2, at 298 K. b The fluorescence quantum yield of Pn. The
fluorescence quantum yields f for ZnP2 and ZnP2·pyrazine were
c
determined to be and 0.0294 and 0.0338, respectively. The absorption
ratio of ZnP2·pyrazine and Pn in the assembly at 564 nm, evaluated from
the spectrum reconstructed by calculation of 4 × ZnP2·pyrazine + Pn.
d The amplifying factor for the assembly (see text and Fig. 6a). e The
fluorescence quenching factor for the ZnP2·pyrazine fluorescence in
the assembly.
which is an index of the initial amounts of the excited porphyrin
components under the present conditions. As expected, the
results indicate the general trend that the longer the alkyl chain
length of Pn, the lower the apparent energy transfer efficiency,
(1−fb), becomes. This point is discussed later in detail. The
interesting relationship, fa  r564 × (1−fb), reported previously
holds also for the present assemblies, indicating that the
observed large enhancement of the fluorescence of Pn results
directly from the absorption of the antenna pigments, ZnP2.4
donor and acceptor porphyrins in the assembly at 564 nm, r564
,
Fig. 6 Fluorescence spectra of the assemblies and their components
in CH2Cl2 (kext = 564 nm). a) The spectra of P9·(ZnP2)4 (4.31 × 10−7 M),
P9 (4.31 × 10−7 M) and ZnP2·pyrazine (1.72 × 10−6 M). The composite
spectrum is almost overlapping with the observed one. b) The spectra
of P1·(ZnP2)4 (7.25 × 10−7 M), P1 (7.25 × 10−7 M) and ZnP2·pyrazine
(2.90 × 10−6 M).
The emission spectra of the assembly constructed on P1 again
show somewhat different behaviour. The fluorescence maxima
of the assembly P1·(ZnP2)4 at 660 and 722 nm are significantly
different from those of uncomplexed P1 at 655 and 718 nm and,
therefore, the notable discrepancy between the reconstructed
spectrum and the observed one arises as shown in Fig. 6b. These
observations clearly indicate that the electronic interaction
between the central porphyrin of P1 and the antenna ZnP2
moieties is not negligible in the assembly and that no simple
relationships between absorption and emission may be expected
for this smallest system. The quenching factor fb obtained as the
mathematical best fit value, however, may be still a fairly good
index for the energy transfer in P1(ZnP2)4, since the fluorescence
component of ZnP2·pyrazine at 609 nm seems to be unaffected
by assembly formation.
Fig. 5 Fluorescence spectroscopic titration of ZnP2 with P9 in CH2Cl2
at 298 K. The assembly is excited at 564 nm; [ZnP2] = 1.82 × 10−6 M,
[P9] = 0, 1.10, 2.17, 3.22, 3.99, 4.81, 5.58, 6.58, 7.56, 8.78 × 10−7 M.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 8 5 2 – 2 8 6 0
2 8 5 5

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Table 2 Fluorescence decay parameters for the porphyrin compo-
Table 3 Kinetic parameters for the fluorescence decay process of the
ZnP2·pyarazine moieties in the assembliesa
nentsa
d
d
Assembly
s0/nsb
kf/ns−1 c
Assembly
s1/nsb
ket/ns−1 c
A0
A1
P1
7.81
8.54
8.36
8.46
8.20
3.20
3.03
0.0146
0.0154
0.0143
0.0142
0.0157
0.0106
0.0097
P1
0.107
0.160
0.474
0.764
1.26
9.03
5.94
1.80
0.996
0.484
0.020
0.023
0.017
0.019
0.019
0.116
0.099
0.052
0.042
0.031
P5
P5
P9
P9
P17
P17
P30
P30
ZnP2·pyrazine
ZnP2
a In CH2Cl2, at 298 K. b The lifetime for the faster component in the
observed biphasic process. The slower lifetime s0 is fixed at 3.2 ns in all
cases. c The rate constant of the intra-assembly energy transfer reaction
calculated from ket = 1/s1−1/s0. d The intensity factors for the slower and
faster decay components of the fluorescence (see text).
a In CH2Cl2, at 298 K. b The lifetime of the porphyrin fluorescence. c The
rate constant for the radiative decay calculated from the relationship
kf = /s0, where  is the fluorescence quantum yield given in Table 1.
Kinetic analyses of the energy transfer reaction in nonameric
porphyrin assemblies
of uncomplexed ZnP2, though the slower lifetime component
seems to be similar to that of ZnP2·pyrazine. Similar biphasic
behaviour is observed for all other assemblies used in the
present work. The results strongly indicate that there are some
energy transfer inactive ZnP2·pyrazine moieties in the assembly,
AZnP2#, which do not equilibrate with the active ones, AZnP2*,
within the time range of the present photochemical processes
(Scheme 3b). Thus, all fluorescence decay data were analysed by
using the biphasic kinetic model,
The dynamic aspects of the energy transfer reaction in the
assemblies are investigated by the method of fluorescence life-
time analyses. At first, the lifetimes of the present porphyrin
components were measured in dichloromethane. All observed
decay profiles for these compounds show a clear, single
exponential fluorescence decay and the resulting lifetimes are
reasonable compared with the typical examples of those of
TPP derivatives. The lifetimes of the component porphyrins
used in this work are tabulated in Table 2 together with the
rate constants of the radiative decay process, kf, calculated
from the lifetimes and the fluorescence quantum yields of the
corresponding porphyrins. All lifetimes of central free base
porphyrins are essentially identical, indicating that the photo-
chemical dynamics of these porphyrins are unaffected by their
alkylpyrazine moieties.
F(t) = A0exp(−t/s0) + A1exp(−t/s1)
(3)
where F is fluorescence intensity, s0 and s1, are lifetimes and A0
and A1 are corresponding intensity factors for the fluorescence
#
of AZnP2 and AZnP2*, respectively. Since it is confirmed
that the longer lifetime s0 is practically equal to that of the
ZnP2·pyrazine complex, 3.2 ns, in all cases the decay curve
analyses were performed by optimising s1 using the fixed s0
value. The energy transfer rate constants ket for the present
assemblies are calculated from the standard equation, ket = 1/
s1−1/s0. The resulting s1, ket and intensity factors A0 and A1 are
summarised in Table 3.
Based on the observed binding constants for the present
assemblies, the concentration of the free ZnP2 is expected to
be negligibly low under the usual conditions of the assembly
construction. Therefore, assuming the simple dynamic model
as shown in Scheme 3a, the decay for the fluorescence from the
A
excited antenna moiety, ZnP2*, in the 1:4 mixture of Pn and
Discussion
ZnP2 is expected to show a single exponential curve, of which
the rate may correspond to the sum of first order rate constants
of the spontaneous decay, kn+kf, and the energy transfer process,
ket. A typical example of the fluorescence decay curve for the
present assembly is shown in Fig. 7, where the fluorescence of
the 1:4 mixture of P9 and ZnP2 is collected in the range from
590 to 620 nm where no fluorescence from P9 appears. In spite of
the above prediction, the observed decay is apparently biphasic.
Since the decay curve is practically unaffected by addition of an
excess of P9, the additional decay process is not attributed to that
The present energy transfer reaction is expected to proceed
via a through-space Förster mechanism. The most direct
confirmation of the mechanism may be obtained from
examination of the relationship between the energy transfer rate
and the donor–acceptor distance. In order to examine this point,
we have adopted two kinds of distance indices for the present
porphyrin assemblies. The first index is a center-to-center
separation distance (Ret) between the zinc porphyrin and the
free base porphyrin estimated from the Förster equation,12
Scheme 3 Photochemical processes for the porphyrin assembly: a) the single kinetic cycle model containing a energy transfer path from homogeneous
antenna porphyrins AZnP2*: b) the parallel kinetic model containing energy transfer inactive antenna porphyrin AZnP2#
.
2 8 5 6
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 8 5 2 – 2 8 6 0

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Table 4 Center-to-center distances (Å) estimated from the Förster
equation [eqn. (4)] and the statistical conformation analysis for the free
alkyl chains
a
b
Assembly
Ret
Rst
P1
12.8
13.7
16.8
18.5
20.8
12.3
14.4
16.0
18.3
21.2
P5
P9
P17
P30
a The distance estimated from Förster’s equation (see text). b The distance
estimated by using the statistical length of alkyl chains (see Fig. 8).
different origins of these two distance indices and apparently
rough simplification for estimation of Rst. The results are also
confirmed more clearly by the logarithmic plot of ket vs. Rst
shown in Fig. 9. The observed linear relationship with the slope
of −6 indicates that the present intra-assembly energy transfer
reactions may mainly proceed via the Förster mechanism.15
Fig. 7 Fluorescence decay profile of P5·(ZnP2)4 in CH2Cl2 collected in
the 590–620 nm range. The sample is excited at 406 nm. The solid line is
calculated by F(t) = 0.023exp(−t/3.2) + 0.16exp(−t/0.16).
(8.8×10−25 )κ2φf JF
ket =
(4)
n4τ0R6et
where j is an orientation factor (j = 2/3 for random
orientation),13 f is the fluorescence quantum yield of the donor
given in the caption of Table 1, n is the solvent refractive index
and JF is the overlap integral obtained from the fluorescence
intensity at wavenumber m (F(m)) of the donor and the molar
extinction coefficient (e(m)) of the Qy absorption band of the
acceptor in the following form,
F(ν)ε(ν)ν−4dν
Fig. 9 Logarithmic plot of Rst vs. ket. The slope of the straight line
is −6. The dashed line shows one possible curve calculated for the Dexter
mechanism using a Bohr radius of ca. 3 Å. See discussion in ref. 15.
∫
JF =
(5)
F(ν)dν
∫
The second distance index is the following end-to-end distance
Since fairly wide range for the energy transfer rate is observed
in the present systematic assembly series, it may be interesting
to inspect the quantitative relationship between the data of the
steady-state fluorescence spectroscopy and the dynamic one.
The energy transfer efficiency evaluated from the fluorescence
L, statistically calculated for the free alkyl chain (CH2)m+1
,
m




1+λ 2λ(1−λ )
1−λ
< L2 >= ml2
−
(6)






m(1−λ)2




ss
spectroscopy, et = 1−fb, and that from the kinetic measure-
dy
where <L2> is the average square end-to-end distance, m is the
number of the bond, l is the bond length of the C–C unit and k
is cos(p−h) for the bond angle h of the alkyl chain.14 The actual
center-to-center distance between the donor and the acceptor
(Rst) statistically estimated is calculated by adding the fixed
length of the rigid aromatic part of the structures to L as shown
in the Fig. 8. The values of Ret and Rst are summarised in Table 4.
There is excellent agreement between Ret and Rst in spite of quite
ment, et = ket/(ko+kf +ket), are tabulated in Table 4. The
discrepancy between these two values found in all assemblies
dy
evidently comes from the fact that the efficiency et does not
take into account the effect of the energy transfer inactive
antenna porphyrin AZnP2#. The total energy transfer efficiency
Tet may be evaluated by using the intensity factors A0 and A1,
which contain the initial concentration and the fluorescence
#
quantum yields for AZnP2 and AZnP2*, 0 = kf/(k0+kf) and
1 = kf/(k0+kf +ket) as follows,
A
φ0[A ZnP# ]init
0
2
=
.
(7)
A
φ [A ZnP* ]init
1
1
2
The total energy transfer efficiency Tet is obtained by using the
intensity ratio,
Tφf
φ0
φ0 −φ
1
Tφet =1−
=
(8)
(A0 / A1 )φ +φ0
1
where Tf is the following total fluorescence quantum yield for
the present assembly:
φ0[A ZnP# ]init +φ [A ZnP# ]init
(1+ A0 / A1 )φφ0
1
Tφf =
=
2
1
2
(9)
Fig. 8 Statistical center-to-center distance, Rst, between the donor and
[A ZnP# ]init +[A ZnP# ]init
(A0 / A1 )φ +φ0
2
2
1
the acceptor connected with the alkyl chain (CH2)n+1
.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 8 5 2 – 2 8 6 0
2 8 5 7

View Article Online
Table 5 Comparison of the apparent and calculated energy transfer
SHIMADZU Multispec-1500 multichannel Photodiode
Array Spectrometer with a Toshiba UV-35 cut-off filter.
Fluorescence spectra were recorded on a HITACHI F-4500
spectrofluorometer. The time-resolved fluorescence lifetime
measurements were performed using a Hamamatsu Photonics
C-4780 instrument with a picosecond light pulser PLP-10 at
406 nm. Samples for the fluorescence lifetime measurements
were used after nitrogen gas bubbling. meso-Tetrakis(2-carboxy-
4-nonylphenyl)porphyrin and its zinc complex (ZnP) were pre-
pared according to our previously reported methods.5
efficiencies
ss
a
dy
b
c
Assembly

et

et
Tet
P1
0.90
0.81
0.76
0.68
0.49
0.96
0.95
0.85
0.76
0.61
0.96
0.93
0.81
0.68
0.48
P5
P9
P17
P30
a The efficiency obtained from the steady-state fluorescence spectro-
scopic data (1−fb). b The apparent efficiency calculated from the energy
transfer rate constant, ket/(k0+kf +ket). c The efficiency corrected for the
effect of energy transfer inactive AZnP2# (see text).
2-(5-Bromopentyl)pyrazine
To a solution of diisopropylamine (1.61 g, 15.9 mmol) in THF
(10 ml) was added dropwise 1.6 M n-BuLi in hexane (10 ml,
15.9 mmol) at −78 °C, with stirring for 1.5 h at this temperature.
2-Methylpyrazine (1.02 g, 10.8 mmol) in THF (5 ml) was added
into the solution at −78 °C. The reddish-purple mixture was
maintained for 3 h at −78 °C with stirring, and then 1,4-
dibromobutane (4.59 g, 21.3 mmol) in THF (5 ml) was added.
The reaction mixture was allowed to warm gradually to room
temperature, and stirring at this temperature was continued
for 16 h. The reaction was quenched by addition of an excess
amount of solid NH4Cl, and the mixture was filtered and
concentrated under reduced pressure. The residue was purified
by column chromatography on silica-gel eluted by hexane to
hexane/ether = 1/1 to afford 1.28 g (5.59 mmol, 53%) of a
T
The values of et calculated by using 0 observed for the
ZnP2·pyrazine complex and 1 evaluated from the kinetic data
for AZnP2* are also tabulated in Table 5. The values of et thus
T
obtained show reasonable agreement with those obtained by the
steady-state fluorescence measurement. In this evaluation, it is
also interesting to note that the energy transfer active antenna
moiety AZnP2* is always the main antenna component in all of
the assemblies, though AZnP2# seems to a play more significant
role in the larger assemblies, i.e. the relative amounts of ZnP2#
A
estimated from eqn. (7) are ca. 0.6, 1, 5, 10 and 24% for P1, P5,
P9, P17 and P30, respectively. The observation that the relative
#
amount of AZnP2 seems to be depend upon the spatial scale
1
colorless oil. H-NMR (200 MHz, CDCl3): d 8.49 (dd, 1H,
of the assembly and not upon the number of the antenna
porphyrins suggests that the observed non-homogeneity of
the antenna moieties has its origin in the size of the degree of
conformational freedom of the assembly.
J = 1.4, 2.4 Hz, 5-pyrz–H), 8.46 (d, 1H, J = 1.4 Hz, 3-pyrz–H),
8.40 (d, 1H, J = 2.4 Hz, 6-pyrz–H), 3.41 (t, 2H, J = 6.8 Hz,
–OCH2–), 2.83 (t, 2H, J = 7.6 Hz, pyrz–CH2–), 2.00–1.70 (m,
4H, –OCH2–CH2– and pyrz–CH2–CH2–), 1.62–1.42 (m, 2H,
–CH2–).
Conclusion
The present assembly systems show a significant antenna effect,
light-condensation and intensification of the excited state of the
specific central porphyrin. The process of the intra-assembly
energy transfer reaction is shown to proceed via a typical
Förster mechanism. The advantage of the self-assembling
methodology is clear not only from the viewpoint of the fact
that the systematic series of porphyrin assemblies are easily
obtained, but also from the fact that the physical properties of
the partial structure in each assembly is easily evaluated by using
the corresponding separated component.
2-(9-Bromononyl)pyrazine
52% (colorless oil). 1H-NMR (200 MHz, CDCl3): d 8.48 (dd, 1H,
J = 1.6, 2.4 Hz 5-pyrz–H), 8.45 (d, 1H, J = 1.6 Hz 3-pyrz–H),
8.38 (d, 1H, J = 2.4 Hz, 6-pyrz–H,), 3.39 (t, 2H, J = 7.1 Hz, –
OCH2–), 2.80 (t, 2H, J = 7.7 Hz, pyrz–CH2–), 1.92–1.64 (m, 4H,
–OCH2–CH2–, pyrz–CH2–CH2–), 1.50–1.16 [m, 10H, –(CH2)5–].
MS (EI): m/z 284, 286 [M]+.
2-(13-Bromotridecanyl)pyrazine
The investigation of the dynamic aspects of the assemblies
unexpectedly reveals that the antenna porphyrins in the
assemblies are not in a homogeneous state. Although the actual
structural properties of the energy transfer inactive antenna site
AZnP2* are unknown, it is interesting to note that the dendritic
multi-porphyrin system also shows similar multi-phasic
fluorescence decay behaviour.16 In order to identify the exact
state of AZnP2#, more detailed analyses of the fast spectroscopic
data, such as consideration of the multi-phase photochemical
process not only for the antenna moiety but also for the acceptor
porphyrin Pn, will be necessary.
Anotherinterestingapplicationof thepresentmulti-porphyrin
assembly is the introduction of a built-in charge separation
system that contains an appropriate electron acceptor moiety.
These developmental investigations are now underway in our
laboratory.
60% (colorless oil). 1H-NMR (200 MHz, CDCl3): d 8.48–8.45 (m,
2H 3 and 5-pyrz–H), 8.40 (d, 1H, J = 2.4 Hz, 6-pyrz–H,), 3.41 (t,
2H, J = 6.6 Hz, –OCH2–), 2.81 (t, 2H, J = 7.3 Hz, pyrz–CH2–),
1.94–1.65 (m, 4H, –OCH2–CH2–, pyrz–CH2–CH2–), 1.48–1.15
[m, 18H, –(CH2)5–]. MS (EI): m/z 340, 342 [M]+.
2-(17-Bromoheptadecanyl)pyrazine
29% (colorless oil). 1H-NMR (200 MHz, CDCl3): d 8.49 (dd, 1H,
J = 1.3, 2.4 Hz, 5-pyrz–H), 8.46 (d, 1H, J = 1.3 Hz, 3-pyrz–H),
8.39 (d, 1H, J = 2.4 Hz, 6-pyrz–H), 3.41 (t, 2H, J = 6.9 Hz,
BrCH2–), 2.62 (t, 2H, J = 8.3 Hz, pyrz–CH2–), 2.02–1.51 (m,
4H, BrCH2–CH2–, pyrz–CH2–CH2–), 1.44–0.79 [m, 26H,
–(CH2)13–].
2-(13-Hydroxyltridecananyl)pyrazine
A mixture of 2-(13-bromotridecanyl)pyrazine (0.27 g, 0.8 mmol)
and potassium acetate (0.8 g, 8.2 mmol) in DMF (11 ml) was
stired for 2 h at 60 °C under nitrogen. After evaporation of the
solvent under reduced pressure the residue was treated with
KOH (500 mg) in methanol (13 ml) at 60 °C. The mixture was
concentrated, and extracted with dichloromethane. The extract
was washed twice by water, and dried over sodium sulfate.
Evaporation of the solvent gave almost pure product (0.19 g,
Experimental
1H-NMR spectra were recorded on a Varian Gemini 200
(200 MHz) or a Brucker ARX500 (500 MHz) spectrometer
in deuteriochloroform or deuteriotetrahydrofuran using the
residual solvent peaks as internal standards of chemical
shift. Low resolution mass spectra were obtained by using
SHIMADZU QP-5000 (EI) or BRUKER REFLEX III
(MALDI-TOF) instruments. High resolution mass spectra
(HRMS) were recorded on a JEOL-700 (FAB) spectro-
meter. Electronic absorption spectra were obtained by using
1
92%) which was used without further purification. H-NMR
(200 MHz, CDCl3): d 8.63–8.36 (m, 3H, pyrz–H), 3.62 (t, 2H,
J = 6.7, –CH2–O), 2.83 (t, 2H, J = 7.9, –CH2–pyrz), 1.86–1.66
2 8 5 8
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 8 5 2 – 2 8 6 0

View Article Online
(m, 2H, –O–CH2CH2–), 1.64–1.45 (m, 2H, pyrz–CH2CH2–),
1.40 (m, 18H, other –CH2–).
1.6 Hz, 3-pyrz–H), 8.37 (d, 4H, J = 2.5 Hz, 6-pyrz–H), 8.08 (d,
8H, J = 8.6 Hz, 2-Ph–H) 7.25 (d, 8H, J = 8.6 Hz, 1-Ph–H), 4.23
(t, 8H, J = 6.6 Hz, Ph–OCH2–), 2.83 (t, 8H, J = 7.6 Hz, pyrz–
CH2–), 1.99–1.93 (m, 8H, PhOCH2–CH2–), 1.82–1.74 (m, 8H,
pyrz–CH2–CH2–), 1.64–1.57 [m, 8H, PhO(CH2)2–CH2–], 1.49–
1.38 [m, 32H, –CH2–(CH2)4–CH2–], −2.76 (brs, 2H, inner N–H).
HRMS (FAB): m/z 1494.8776 [M]+. Calc. for C96H110O4N12:
1494.8773 (D = +0.2 ppm). UV-Vis (CH2Cl2): kmax/nm (loge) 422
(5.54), 519 (4.10), 556 (3.97), 595 (3.59), 651 (3.67).
2-(30-Bromo-17-oxatricontane)pyrazine
2-(13-Hydroxyltridecananyl)pyrazine (133 mg, 0.48 mmol)
dissolved in THF (5 ml) was treated with NaH (60%, 100 mg)
under nitrogen, and was reacted with 1,16-dibromohexadecane
(371 mg, 0.97 mmol) in THF (3 ml) under reflux for 16 h. The
reaction mixture was quenched by methanol, and concentrated
under reduced pressure. The residue was dissolved in dichloro-
methane, washed with water, and dried over sodium sulfate. The
crude products were purified by silica-gel chromatography eluted
with hexane–diethyl ether (100/0 to 40/60) to afford 122 mg
Tetrakis[p-{17-(2-pyrazinyl)-n-heptadecanoxy}phenyl]porphyrin
(P17)
1
29%. H-NMR (500 MHz, CDCl3): d 8.84 (s, 8H, pyrrole–H),
1
(45%) of product. H-NMR (200 MHz, CDCl3): d 8.5–8.4 (m,
8.45 (dd, 4H, J = 1.4, 2.5 Hz, 5-pyrz–H), 8.42 (d, 4H, J =
1.4 Hz, 3-pyrz–H), 8.35 (d, 4H, J = 2.5 Hz, 6-pyrz–H), 8.09
(d, 8H, J = 8.5 Hz, 2-Ph–H), 7.26 (d, 8H, J = 8.5 Hz, 1-Ph–H),
4.23 (t, 8H, J = 6.5 Hz, Ph–OCH2–), 2.77 (t, 8H, J = 7.8 Hz,
pyrz–CH2–), 2.00–1.93 (m, 8H, PhOCH2–CH2–), 1.75–1.68
(m, 8H, pyrz–CH2–CH2–), 1.64–1.55 (m, 8H, –CH2–CH2–
CH2–), 1.49–1.42 (m, 8H, –CH2–CH2–CH2–), 1.42–1.23 [m,
88H, –(CH2)11–], −2.77 (brs, 2H, inner N–H); HRMS (FAB):
m/z 1943.3735 [M]+. Calc. for C128H174O4N12: 1943.3781
(D = −2.4 ppm). UV-Vis (CH2Cl2) kmax/nm (loge) 422 (5.54), 519
(4.11), 557 (3.98), 595 (3.60), 652 (3.76).
3H, pyrz–H), 3.5–3.3 (m, 6H, Br–CH2–, –CH2–O–CH2), 2.82 (t,
2H, J = 7.8 Hz, pyrz–CH2–), 1.86–1.11 (m, 48H, CH2).
2-(Bromomethyl)pyrazine
A solution of N-bromosuccinimide (3.78 g, 21.2 mmol),
AIBN (3.43 g, 20.9 mmol), and 2-methylpyrazine (2.00 g,
21.2 mmol) in benzene (15 ml) was refluxed for 1 h. After cool-
ing in an ice-bath, the mixture was filtered, and the solvent was
evaporated under reduced pressure. The residue was chromato-
graphed on silica-gel eluted with hexane/diethyl ether to afford
almost pure product, which was immediately used for the next
porphyrin synthesis since it was unstable at room temperature
Tetrakis[p-{30-(2-pyrazinyl)-17-oxatricontanoxy}phenyl]-
porphyrin (P30)
1
under air. H-NMR (200 MHz, CDCl3): d 8.71 (d, 1H, J =
1.4 Hz, 3-pyrz–H), 8.54 (dd, 1H, J = 1.4, 2.5 Hz, 5-pyrz–H),
8.49 (d, 1H, J = 2.5 Hz, 6-pyrz–H), 4.55 (s, 2H, –CH2–). MS
(EI): m/z 172, 174 [M]+.
Preparative HPLC (ODS) eluted with methanol followed by
recrystallization from dichloromethane/methanol gave 3.4 mg
(3%) product. H-NMR (500 MHz, CD2Cl2): d 8.93 (s, 8H,
1
pyrrole–H), 8.46 (dd, 4H, J = 1.5, 2.5 Hz, 5-pyrz–H), 8.44 (d,
4H, J = 1.5 Hz, 3-pyrz–H), 8.36 (d, 4H, J = 2.5 Hz, 6-pyrz–H),
8.14 (d, 8H, J = 6.5 Hz, 2-Ph–H), 7.33 (d, 8H, J = 6.5 Hz,
1-Ph–H), 4.30 (t, 8H, J = 6.5 Hz, Ph–OCH2–), 3.39–3.37 (m,
16H, –CH2–OCH2–), 2.80 (t, 8H, J = 7.8 Hz, pyrz–CH2–),
2.05–1.99 (m, 8H, PhOCH2–CH2–), 1.77–1.71 (m, 8H, pyrz–
CH2–CH2–), 1.70–1.64 (m, 8H, –CH2–CH2–CH2–), 1.58–1.23
(m, 176H, other –CH2–), −2.77 (brs, 2H, inner N–H); HRMS
(FAB): m/z 2680.1079 [M]+. Calc. for C176H270O8N12: 2680.1090
(D = −0.4 ppm). UV-Vis (CH2Cl2) kmax/nm (loge) 422 (5.60), 519
(4.16), 557 (4.04), 595 (3.67), 652 (3.77).
Tetrakis[p-{5-(2-pyrazinyl)-n-pentoxy}phenyl]porphyrin (P5)
A mixture of tetrakis(p-hydroxyphenyl)porphyrin (113.5 mg,
0.167 mmol), 2-(5-bromo-n-pentyl)pyrazine (475.8 mg,
2.08 mmol), K2CO3 (476.1 mg, 3.44 mmol), and 18-crown-6
(93.4 mg, 0.353 mmol) in DMF (10 ml) was stirred at 60 °C
under N2 overnight. The reaction mixture was cooled to room
temperature, and the solvent was evaporated under reduced
pressure. The residue was dissolved in CH2Cl2, washed with
water, and dried over sodium sulfate. Filtration, followed by
evaporation of the solvent gave a crude product as a black solid
which was twice purified by column chromatography on silica-
gel eluted with CH2Cl2/MeOH (100/0 to 95/5), and recrystallized
References
1
from CH2Cl2/MeOH to afford 108 mg (0.085 mmol, 51%). H-
1 J. Deisenhofer, O. Epp, K. Miki, R. Huber and H. Michel,
Nature, 1985, 318, 618; G. McDermott, S. M. Prince, A. A. Freer,
A. M. Hawthornthwaite-Lawless, M. Z. Papiz, R. J. Cogdell and
N. W. Isaacs, Nature, 1995, 374, 517; T. Pullerits and V. Sundstrom,
Acc. Chem. Res., 1996, 29, 381.
NMR (500 MHz, CDCl3): d 8.83 (s, 8H, pyrrole), 8.53–8.51 (m,
8H, 3 and 5-pyrz–H), 8.42 (d, 4H, J = 2.3 Hz, 6-pyrz–H), 8.08
(d, 8H, J = 8.1 Hz, 2-Ph–H) 7.24 (d, 8H, J = 8.1 Hz, 1-Ph–H),
4.25 (t, 8H, J = 6.0 Hz, Ph–OCH2–), 2.52 (t, 8H, J = 7.9 Hz,
pyrz–CH2–), 2.06–1.99 (m, 8H, PhOCH2–CH2–), 1.99–1.91 (m,
8H, PyCH2–CH2–), 1.79–1.68 (m, 8H, –CH2–CH2–CH2–), −2.77
(brs, 2H, inner N–H); HRMS (FAB): m/z 1270.6283 [M]+. Calc.
for C80H78O4N12: 1270.6269 (D = +1.1 ppm). UV-Vis (CH2Cl2):
kmax/nm (loge) 422 (5.62), 519 (4.15), 556 (4.01), 595 (3.64), 651
(3.70).
2 For examples of recent developments, see: M. Wasielewski, Chem.
Rev., 1992, 92, 435; D. Gust, T. A. Moore and A. L. Moore, Acc.
Chem. Res., 1993, 26, 198; H. Kurreck and M. Fuber, Angew.
Chem., Int. Ed. Engl., 1995, 34, 849; A. Harriman and J.-P. Sauvage,
Chem. Soc. Rev., 1996, 24, 41; V. Balzani, A. Juris, M. Venturi,
S. Champagna and S. Serroni, Chem. Rev., 1996, 96, 759; M.-J.
Blanco, M. C. Jiménez, J.-C. Chambron, V. Heitz, M. Linke
and J.-P. Sauvage, Chem. Soc. Rev., 1999, 28, 293; D. Gust,
T. A. Moore and A. L. Moore, Acc. Chem. Res., 2001, 34, 40;
M.d. R. Benitetes, T. E. Johnson, S. Weghorn, L. Yu, P. D. Rao,
J. R. Diers, S. I. Yang, C. Kimaier, D. F. Bocian, D. Holte and
J. S. Lindsey, J. Mater. Chem., 2002, 12, 65; V. Balzani, A. Credi,
M. Venturi, Molecular Devices and Machines – A Journey into the
Nano World, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim,
2003.
3 For recent reviews of the porphyrin assemblies, see: H. Ogoshi,
T. Mizutani, T. Hayashi and Y. Kuroda, in The Porphyrin Hand-
book. Vol.6, Applications: Past, Present and Future, K. M. Kadish,
K. M. Smith and R. Guilard, ed., Academic Press, New York,
2000, pp. 279–340; and A. K. Burell, D. L. Officer, P. G. Plieger
and D. C. W. Reid, Chem. Rev., 2001, 101, 2751. For examples of
trimeric or lager porphyrin assemblies, see: P. Tecilla, R. P. Dixon,
G. Slobodkin, D. S. Alavi, D. H. Waldeck and A. D. Hamilton,
J. Am. Chem. Soc., 1990, 112, 9408; A. M. Brun, S. J. Atherton,
A. Harriman, V. Heitz and J.-P. Sauvage, J. Am. Chem. Soc., 1992,
Tetrakis{p-(2-pyrazinyl)methoxyphenyl}porphyrin (P1)
1
9%. H-NMR (500 MHz, CDCl3): d 9.06 (d, 4H, J = 1.5 Hz,
3-pyrz–H), 8.84 (s, 8H, pyrrole–H), 8.66 (dd, 4H, J = 1.5,
2.4 Hz, 5-pyrz–H), 8.63 (d, 4H, J = 2.4 Hz, 6-pyrz–H), 8.14
(d, 8H, J = 8.2 Hz, Ph–H), 7.38 (d, 8H, J = 8.2 Hz, Ph–H),
5.33 (s, 8H, –CH2–), −2.79 (brs, 2H, inner N–H). HRMS
(FAB): m/z 1046.3771 [M]+. Calc. for C64H46O4N12: 1046.3765
(D = +0.6 ppm). UV-Vis (CH2Cl2): kmax/nm (loge) 421 (5.66), 518
(4.21), 554 (4.02), 595 (3.69), 650 (3.74).
Tetrakis[p-{9-(2-pyrazinyl)-n-nonoxy}phenyl]porphyrin (P9)
1
13%. H-NMR (500 MHz, CDCl3): d 8.84 (s, 8H, pyrrole–H),
8.48 (dd, 4H, J = 1.6, 2.5 Hz, 5-pyrz–H), 8.46 (d, 4H, J =
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 8 5 2 – 2 8 6 0
2 8 5 9

View Article Online
114, 4632; F. Odobel and J.-P. Sauvage, New J. Chem., 1994, 18,
1139; S. Anderson, H. L. Anderson, A. Bashall, M. Mcpartlin
and J. K. M. Sanders, Angew.Chem., Int. Ed. Engl., 1995, 34,
1096; A. V. Chernook, U. Rempel, C. von Borczyskowski,
A. M. Shulgam and E. I. Zenkevich, Chem. Phys. Lett., 1996,
254, 229; C. A. Hunter and R. K. Hyde, Angew.Chem., Int. Ed.
Engl., 1996, 35, 1936; C. M. Drain, K. C. Russell and J.-M. Lehn,
Chem. Commun., 1996, 337; Y. Kuroda, Y. Kato and H. Ogoshi,
Chem. Commun., 1997, 469; Y. Kuroda, N. Shiraishi, K. Sugou,
K. Sasaki and H. Ogoshi, Tetrahedron Lett., 1998, 39, 2993;
P. N. Taylor and H. L. Anderson, J. Am. Chem Soc., 1999, 121,
11538; R. Takahashi and Y. Kobuke, J. Am. Chem. Soc., 2003, 125,
2372.
4 Y. Kuroda, K. Sugou and K. Sasaki, J. Am. Chem. Soc., 2000, 122,
7833; K. Sugou, K. Sasaki, K. Kitajima, T. Iwaki and Y. Kuroda, J.
Am. Chem. Soc., 2002, 124, 1182.
5 Y. Kuroda, A. Kawashima, T. Urai and H. Ogoshi, Tetrahedron
Lett., 1995, 36, 8449; Y. Kuroda, A. Kawashima and H. Ogoshi,
Chem. Lett., 1996, 57; Y. Kuroda, A. Kawashima, Y. Hayashi and
H. Ogoshi, J. Am. Chem. Soc., 1997, 119, 4929.
10 Since the larger value of the binding constant gives practically the
same theoretical curve fitting, the value determined by the present
titration is regarded as a minimum estimate.
11 J. A. Blackburn, Anal. Chem., 1965, 37, 1000.
12 T. Förster, Fluorenzenz Organische Verbindungen, Vandenhoech
and Ruprech, Göttingen, 1951; T. Förster, Ann. Physik, 1948, 2,
55; T. Förster, Discuss. Faraday Soc., 1959, 27, 7. For an example of
practical application for the energy transfer between porphyrins, see:
J. L. Sessler, B. Wang and A. Harriman, J. Am. Chem. Soc., 1995,
117, 704.
13 The orientation factor should not be necessarily fixed as 2/3 owing
to the motional constraints for the donor and acceptor in the present
assemblies which is suggested by the existence of AZnP2#. This value
was tentatively used for following reasons: (a) the antenna moiety
#
which has an extremely unfavourable orientation such as AZnP2
is expected to be minor; (b) a variation of the orientation factor
of medium values results in minor error in the estimation of the
distance because of the sixth root relationship, see, for example:
B. Valeur, Molecular Fluorescence, Wiley-VCH Verlag GmbH,
Weinheim, 2002.
6 Y. Houminer, R. A. Fenner, H. V. Secor and J. I. Seemann, J. Org.
Chem., 1987, 52, 3971.
7 The analyses and manipulation of the electronic and the fluorescence
spectra in this work were performed by using the software SPANA
developed in this laboratory, which is available from our web-site,
http://www.poly.kit.ac.jp/kinosekk/index.html.
8 The weak electronic interactions for these assemblies are observed in
the Soret band region, where the observed spectra of the Pn·(ZnP2)4
assemblies tend to broaden compared with those composed of the
corresponding component spectra. For the spectroscopic effect
of the multi-porphyrin interaction, see, for example: T. Nagata,
A. Osuka and K. Maruyama, J. Am. Chem. Soc., 1990, 112, 3054;
A. Osuka, N. Tanabe, S. Nakajima and K. Maruyama, J. Chem.
Soc., Perkin Trans. 2, 1996, 199.
14 P. J. Flory, Statistical Mechanics of Chain Molecules, John Wiley &
Sons, Inc., New York, 1969.
15 During the examination of this article, one of the referees suggests
that the Dexter mechanism may already have some weight in the
energy transfer at a distance of 12–14 Å. If the P1 point in Fig. 9
is neglected due to the observed orbital perturbation, the upward
deviation observed for the P5 assembly may be explained by the
contribution of the Dexter mechanism as shown by the dashed line in
Fig. 9. This attractive possibility is important for the further detailed
investigation of assemblies which have the donor–acceptor distance
in the transitional region for the Dexter and Förster mechanisms.
See, for related discussions: P. Brodard, S. Matzinger, E. Vauthey,
O. Mongin, C. Papamicael and A. Gossauer, J. Phys. Chem. A, 1999,
103, 5858; S. Faure, C. Stern, R. Guilard and P. D. Harvey, J. Am.
Chem. Soc., 2004, 126, 1253.
9 For the theoretical treatment of the multiple equilibrium processes,
see: C. Tanford, Physical Chemistry of Macromolecules, John Wiley
& Sons, New York, 1961.
16 M.-S. Choi, T. Aida, T. Yamazaki and I. Yamazaki, Chem. Eur. J,
2002, 8, 2667.
2 8 6 0
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 8 5 2 – 2 8 6 0